Essential Cell Biology 5th edition

Stem Cells and Tissue Renewal
713
LUMEN OF GUT
epithelial-cell migration
from birth at the bottom
of the crypt to loss at the
top of the villus
(transit time in
humans is
3–6 days)
LUMEN
OF GUT
villus (no cell division)
cross section
of villus
villus
absorptive
brush-border
cells
mucussecreting
goblet cells
connective
tissue
crypt
epithelial
cells
crypt
loose
connective
tissue
absorptive cell
secretory cell
direction of
cell movement
nondividing,
terminally
differentiated
cells
dividing
precursor cells
cross
section
of crypt
(A)
100 µm
(B)
dividing
stem cells
nondividing, terminally
differentiated Paneth cells
Figure 20–35 Renewal occurs continuously in the epithelial lining of the adult mammalian intestine.
(A) Micrograph of a section of part of the lining of the small intestine showing the villi and crypts. Mucus-secreting
goblet cells (stained purple) are interspersed among the absorptive brush-border cells in the epithelium covering
the villi. Smaller numbers of two other secretory cell types—enteroendocrine cells (not visible here), which secrete
gut hormones, and Paneth cells, which secrete antibacterial proteins—are also present and derive from the same
stem cells. (B) Drawings showing the pattern of cell turnover and the proliferation of stem cells and precursor cells.
The stem cells (red) give rise ECB5 mainly e20.36-20.36 to proliferating precursor cells (yellow), which slide continuously upward and
terminally differentiate into secretory (purple) or absorptive (blue) cells, which are shed from the tip of the villus.
The stem cells also give rise directly to terminally differentiated Paneth cells (gray), which move down to the
bottom of the crypt.
The pattern of cell replacement varies from one stem-cell-based tissue to
another. In the lining of the small intestine, for example, the absorptive
and secretory cells are arranged as a single-layered, simple epithelium
covering the surfaces of the fingerlike villi that project into the gut lumen.
This epithelium is continuous with the epithelium lining the crypts, which
descends into the underlying connective tissue (Figure 20–35A). The
stem cells lie near the bottom of the crypts, where they give rise mostly
to proliferating precursor cells, which move upward in the plane of the
epithelial sheet. As they move upward, the precursor cells terminally differentiate
into absorptive or secretory cells, which are shed into the gut
lumen and die when they reach the tips of the villi (Figure 20–35B).
A contrasting example is the epidermis, a stratified epithelium. In the epidermis,
proliferating stem cells and precursor cells are confined to the
basal layer, adhering to the basal lamina. The differentiating cells travel
outward from their site of origin in a direction perpendicular to the plane
of the cell sheet; terminally differentiated cells and their corpses are
eventually shed from the skin surface (Figure 20–36).
Often, a single type of stem cell gives rise to several types of differentiated
progeny: the stem cells of the intestine, for example, produce absorptive
cells, goblet cells, and several other secretory cell types. The process of
QUESTION 20–7
Why do you suppose epithelial cells
lining the gut are lost and replaced
(renewed) frequently, whereas most
neurons last for the lifetime of the
organism?